The present invention relates to a device for automatically adjusting an output voltage within a predetermined range even when an input voltage is increased or decreased beyond a predetermined range in an electric power saving device, such as an autotransformer, so that an abnormal voltage is not supplied to a load.
Japanese Patent Application No.1804/1995, which was filed prior to the present application by the same applicant, of which U.S. Pat. No. 5,654,627 is a U.S. counterpart is one of known conventional electric power saving apparatuses.
In a conventional electric power saving apparatus, as shown in FIG. 4, plural sets of exciting coils L3, L4, L7 and L8, which are wound on a single-phase core-type core (9) in a mutually connected series, are connected between the ends of plural sets of main coils L1, L2, L5 and L6 which are phase-wound on the core (9), in which the combinations of connection of the exciting coils L3, L4, L7 and L8 are controlled by thyristors (1, 2, 3, 4, 5, 6, 7, 8) connected with a voltage sensor (not shown) provided at the input ends of the main coils L2 and L6, whereby an output voltage is prevented from an unnecessary increase and decrease.
For example, a circuit, on which the exciting coils are combined to cause the voltage to output by dropping by 6V for an input voltage of 100V, is automatically changed to another circuit which has a lower-voltage rate of 3V and then 0V when the input voltage is decreased. On the other hand, the circuit will be automatically changed to another circuit which has a upper-voltage rate of 3V or 6V when the input voltage is too low. Therefore, the output voltage is prevented from unnecessarily increasing and decreasing so as to keep the effect on the load as little as possible.
However, conventional electric power saving transformers with automatic voltage changing device such as the aforementioned example are for single-phase current, and are restricted to autotransformers of single-phase three-wire or two-wire system, except for a power generator.
In the case of three-phase current, a simultaneous ON/OFF change of voltages of the three phases is desired. Since the three phases of alternating current are not at the same voltage level and simultaneous OFF is impossible for thyristors as electric switches, the control circuit is complex and difficult to manage compared with for single-phase current. Accordingly, manual voltage change has been performed conventionally instead of automatic voltage change in three-phase current.